Electronic metering of electrical energy is a mature field of technology, and today's metering products must minimize hardware cost to be competitive. Typically, one of the major cost elements in a current or Watt-hour meter is the current transformer responsible for accurately reproducing the waveforms of the current to be measured. Normally, the current to be measured is the current component at the power frequency, 60 Hz for systems in the United States and 50 Hz for many international systems. However, in addition to the power frequency currents, there are harmonic currents and DC currents that can be present. These harmonic and DC currents can adversely affect the performance of current transformers designed to operate at the power frequency. This is a particularly serious concern where extremely accurate billing information is required.
Previous metering technologies have used a variety of current sensing techniques. Generally, these techniques offer a compromise of accuracy for power frequency current with and without DC current present. The DC current tends to saturate magnetic materials, and consequently it adversely affects accuracy. Typical current sensing materials, such as Supermalloy, tend to be expensive and are priced proportional to weight or volume. Accordingly, a consequence of the presence of DC current is the need for more magnetic material to obtain the same AC accuracy, which in turn results in higher costs. Therefore, a need exists for a current measurement technique that will improve power frequency metering accuracy in the presence of DC current.